In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of applications; the nitride ion, N3−, is never encountered in protic solution because it is so basic that it would be protonated immediately. Its ionic radius is estimated to be 140 pm. Like carbides, nitrides are refractory materials owing to their high lattice energy which reflects the strong attraction of "N3−" for the metal cation. Thus, titanium nitride and silicon nitride are used as cutting hard coatings. Hexagonal boron nitride, which adopts a layered structure, is a useful high-temperature lubricant akin to molybdenum disulfide. Nitride compounds have large band gaps, thus nitrides are insulators or wide bandgap semiconductors; the wide band gap material gallium nitride is prized for emitting blue light in LEDs. Like some oxides, nitrides can absorb hydrogen and have been discussed in the context of hydrogen storage, e.g. lithium nitride.
Classification of such a varied group of compounds is somewhat arbitrary. Compounds where nitrogen is not assigned −3 oxidation state are not included, such as nitrogen trichloride where the oxidation state is +3. Only one alkali metal nitride is stable, the purple-reddish lithium nitride, which forms when lithium burns in an atmosphere of N2. Sodium nitride remains a laboratory curiosity; the nitrides of the alkaline earth metals have the formula. Examples include Be3N2, Mg3N2, Ca3N2, Sr3N2; the nitrides of electropositive metals hydrolyze upon contact with water, including the moisture in the air: Mg3N2 + 6 H2O → 3 Mg2 + 2 NH3 Boron nitride exists as several forms. Nitrides of silicon and phosphorus are known, but only the former is commercially important; the nitrides of aluminium and indium adopt diamond-like wurtzite structure in which each atom occupies tetrahedral sites. For example, in aluminium nitride, each aluminium atom has four neighboring nitrogen atoms at the corners of a tetrahedron and each nitrogen atom has four neighboring aluminium atoms at the corners of a tetrahedron.
This structure is like hexagonal diamond. Thallium nitride, Tl3N is known, but thallium nitride, TlN, is not. For the group 3 metals, ScN and YN are both known. Group 4, 5, 6 transition metals, the titanium and chromium groups all form nitrides, they are chemically stable. Representative is titanium nitride. Sometimes these materials are called "interstitial nitrides." Nitrides of the Group 7 and 8 transition metals decompose readily. For example, iron nitride, Fe2N decomposes at 200 °C. Platinum nitride and osmium nitride may contain N2 units, as such should not be called nitrides. Nitrides of heavier members from group 11 and 12 are less stable than copper nitride, Cu3N and Zn3N2: dry silver nitride is a contact explosive which may detonate from the slightest touch a falling water droplet. Many metals form molecular nitrido complexes; the main group elements form some molecular nitrides. Cyanogen and tetrasulfur tetranitride are rare examples of a molecular binary nitrides, they dissolve in nonpolar solvents.
Both undergo polymerization. S4N4 is unstable with respect to the elements, but less so that the isostructural Se4N4. Heating S4N4 gives a polymer, a variety of molecular sulfur nitride anions and cations are known. Related to but distinct from nitride is pernitride, N2−2
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform. The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, cold rolling processes the most tonnage out of all cold working processes. Roll stands holding pairs of rolls are grouped together into rolling mills that can process metal steel, into products such as structural steel, bar stock, rails. Most steel mills have rolling mill divisions that convert the semi-finished casting products into finished products. There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, controlled rolling.
The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. The earliest rolling mills in crude form but the same basic principles were found in Middle East and South Asia as early as 600 BCE. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590; these passed flat bars between rolls to form a plate of iron, passed between grooved rolls to produce rods of iron. The first experiments at rolling iron for tinplate took place about 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll'Pontypool plates'—blackplate; this began to be rerolled and tinned to make tinplate. The earlier production of plate iron in Europe had been in forges, not rolling mills; the slitting mill was adapted to producing hoops and iron with a half-round or other sections by means that were the subject of two patents of c. 1679. Some of the earliest literature on rolling mills can be traced back to Christopher Polhem in 1761 in Patriotista Testamente, where he mentions rolling mills for both plate and bar iron.
He explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time. A patent was granted to Thomas Blockley of England in 1759 for the rolling of metals. Another patent was granted in 1766 to Richard Ford of England for the first tandem mill. A tandem mill is one. Rolling mills for lead seem to have existed by the late 17th century. Copper and brass were rolled by the late 18th century. Modern rolling practice can be attributed to the pioneering efforts of Henry Cort of Funtley Iron Mills, near Fareham, England. In 1783, a patent was issued to Henry Cort for his use of grooved rolls for rolling iron bars. With this new design, mills were able to produce 15 times more output per day than with a hammer. Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at the time, thus modern writers have called him "father of modern rolling."
The first rail rolling mill was established by John Birkenshaw in 1820, where he produced fish bellied wrought iron rails in lengths of 15 to 18 feet. With the advancement of technology in rolling mills, the size of rolling mills grew along with the size of the products being rolled. One example of this was at The Great Exhibition in 1851, where a plate 20 feet long, 3 ½ feet wide, 7/16 of an inch thick, weighing 1,125 pounds, was exhibited by the Consett Iron Company. Further evolution of the rolling mill came with the introduction of three-high mills in 1853 used for rolling heavy sections. Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening; the starting material is large pieces of metal, like semi-finished casting products, such as slabs and billets. If these products came from a continuous casting operation the products are fed directly into the rolling mills at the proper temperature.
In smaller operations, the material must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces; as the material is worked, the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling. Hot-rolled metals have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm thick have some directional properties. Non-uniform cooling will induce a lot of residual stresses, which occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, an oxide that forms at high temperatures, it is removed via pickling or the smooth clean surface process, which reveals a smooth surface.
Dimensional tolerances are us
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to stopping corrosion. In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion; this type of damage produces oxide or salt of the original metal, results in a distinctive orange colouration. Corrosion can occur in materials other than metals, such as ceramics or polymers, although in this context, the term "degradation" is more common. Corrosion degrades the useful properties of materials and structures including strength and permeability to liquids and gases. Many structural alloys corrode from exposure to moisture in air, but the process can be affected by exposure to certain substances.
Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces; as a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less predictable. Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte, or when the same metal is exposed to electrolyte with different concentrations. In a galvanic couple, the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a slower rate; when immersed separately, each metal corrodes at its own rate. What type of metal to use is determined by following the galvanic series. For example, zinc is used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the marine industry and anywhere water contacts pipes or metal structures.
Factors such as relative size of anode, types of metal, operating conditions affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is prevented by the use of sacrificial anodes. In any given environment, one metal will be either more noble or more active than others, based on how its ions are bound to the surface. Two metals in electrical contact share the same electrons, so that the "tug-of-war" at each surface is analogous to competition for free electrons between the two materials. Using the electrolyte as a host for the flow of ions in the same direction, the noble metal will take electrons from the active one; the resulting mass flow or electric current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is useful in predicting and understanding corrosion, it is possible to chemically remove the products of corrosion. For example, phosphoric acid in the form of naval jelly is applied to ferrous tools or surfaces to remove rust.
Corrosion removal should not be confused with electropolishing, which removes some layers of the underlying metal to make a smooth surface. For example, phosphoric acid may be used to electropolish copper but it does this by removing copper, not the products of copper corrosion; some metals are more intrinsically resistant to corrosion than others. There are various ways of protecting metals from corrosion including painting, hot dip galvanizing, combinations of these; the materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any corrosion products of gold or platinum tend to decompose spontaneously into pure metal, why these elements can be found in metallic form on Earth and have long been valued. More common "base" metals can only be protected by more temporary means; some metals have slow reaction kinetics though their corrosion is thermodynamically favorable. These include such metals as zinc and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate.
An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is immune to electrochemical corrosion under normal conditions. Passivation refers to the spontaneous formation of an ultrathin film of corrosion products, known as a passive film, on the metal's surface that act as a barrier to further oxidation; the chemical composition and microstructure of a passive film are different from the underlying metal. Typical passive film thickness on aluminium, stainless steels, alloys is within 10 nanometers; the passive film is different from oxide layers that are formed upon heating and are in the micrometer thickness range – the passive film recovers if removed or damaged whereas the oxide layer does not. Passivation in natural environments such as air and soil at moderate pH is seen in such materials as aluminium, stainless steel and silicon. Passivation is determined by metallurgical and environmental factors; the effect of pH is summarized using Pourbaix diagrams.
Some conditions that inhibit passivation include high pH for aluminium and zinc, low pH or the p
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, cooling. In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness; as the material cools it recrystallizes. For many alloys, including carbon steel, the crystal grain size and phase composition, which determine the material properties, are dependent on the heating, cooling rate. Hot working or cold working after the annealing process alter the metal structure, so further heat treatments may be used to achieve the properties required. With knowledge of the composition and phase diagram, heat treatment can be used to adjust between harder and more brittle, to softer and more ductile.
In the cases of copper, steel and brass, this process is performed by heating the material for a while and slowly letting it cool to room temperature in still air. Copper and brass can be cooled in air, or by quenching in water, unlike ferrous metals, such as steel, which must be cooled to anneal. In this fashion, the metal is softened and prepared for further work—such as shaping, stamping, or forming. Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat increases the rate of diffusion by providing the energy needed to break bonds; the movement of atoms has the effect of redistributing and eradicating the dislocations in metals and in ceramics. This alteration to existing dislocations allows a metal object to deform more increasing its ductility; the amount of process-initiating Gibbs free energy in a deformed metal is reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed stress relief.
The relief of internal stresses is a thermodynamically spontaneous process. The high temperatures at which annealing occurs serve to accelerate this process; the reaction that facilitates returning the cold-worked metal to its stress-free state has many reaction pathways involving the elimination of lattice vacancy gradients within the body of the metal. The creation of lattice vacancies is governed by the Arrhenius equation, the migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion. In steel, there is a decarburation mechanism that can be described as three distinct events: the reaction at the steel surface, the interstitial diffusion of carbon atoms and the dissolution of carbides within the steel; the three stages of the annealing process that proceed as the temperature of the material is increased are: recovery, recrystallization, grain growth. The first stage is recovery, it results in softening of the metal through removal of linear defects called dislocations and the internal stresses they cause.
Recovery occurs at the lower temperature stage of all annealing processes and before the appearance of new strain-free grains. The grain size and shape do not change; the second stage is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has completed grain growth occurs. In grain growth, the microstructure starts to coarsen and may cause the metal to lose a substantial part of its original strength; this can however be regained with hardening. The high temperature of annealing may result in oxidation of the metal’s surface, resulting in scale. If scale must be avoided, annealing is carried out in a special atmosphere, such as with endothermic gas. Annealing is done in forming gas, a mixture of hydrogen and nitrogen; the magnetic properties of mu-metal are introduced by annealing the alloy in a hydrogen atmosphere. Large ovens are used for the annealing process; the inside of the oven is large enough to place the workpiece in a position to receive maximum exposure to the circulating heated air.
For high volume process annealing, gas fired conveyor furnaces are used. For large workpieces or high quantity parts, car-bottom furnaces are used so workers can move the parts in and out. Once the annealing process is completed, workpieces are sometimes left in the oven so the parts cool in a controllable way. While some workpieces are left in the oven to cool in a controlled fashion, other materials and alloys are removed from the oven. Once removed from the oven, the workpieces are quickly cooled off in a process known as quench hardening. Typical methods of quench hardening materials involve media such as air, oil, or salt. Salt is used as a medium for quenching in the form of brine. Brine provides faster cooling rates than water; this is because when an object is quenched in water steam bubbles form on the surface of the object reducing the surface area the water is in contact with. The salt in the brine reduces the formation of steam bubbles on the object's surface, meaning there is a larger surface area of the object in contact with the water, providing faster cooling rates.
Quench hardening is applicable to some ferrous alloys, but not copper alloys. In the semiconductor industry, silicon wafers are annealed, so that dopant atoms boron, phosphorus or arsenic, can diffuse into substitutional positions in the cr